The invention relates to a multisite network of several single radio frequency (RF) broadcast site systems. Digital trunked radio transceivers capable of handling communications between numerous mobile units and dispatcher consoles in a single geographic area are known. Trunked RF repeater systems are used, for example, by public service organizations, such as police, fire and rescue departments, taxi services and commercial delivery fleets. These RF repeater systems permit a relatively-limited number of RF communications channels to be shared by a large number of users--while providing relative privacy to any particular RF communication (conversation). Typical state-of-the-art RF repeater systems are "digitally trunked" and use digital signals conveyed over the RF channels (in conjunction with digital control elements connected in the system) to accomplish "trucking" (time-sharing) of the limited number of RF channels among a large number of users. A multisite switch links several individual RF broadcast site trunked systems to form a wide-area network in which radio users in one site area can communicate with users in another site area.
The sharing of a limited number of radio channels between individual sites in a multisite networks gives rise to problems that occur when transmissions from one site are received and misinterpreted by a receiver in another site. For example, a transmission from a mobile radio unit in one site area and intended for the repeating base station in that site may be inadvertently received by the repeating station at an adjacent site. While the adjacent site should not receive or act on a transmission from a mobile unit in another site, on rare occasions adjacent sites have improperly interpreted such remote transmissions as legitimate transmissions from their own site area. When a site repeating station acts on a transmission from another site, the repeating station may, for example, incorrectly interpret the transmission as an emergency message from its site area and pass the emergency message to a dispatcher console. The dispatcher sees an emergency message for which there is no emergency or even a corresponding mobile unit in the site area from which the erroneous message originated. Depending on the content of the message, e.g., a mobile unit emergency, the dispatcher may act on the message and commit resources, such as police or rescue units, to a particular site location, even though there is no actual reason to commit those resources.
Similarly, a transmission processed by the wrong site repeating station may cause the station to incorrectly log the mobile unit as having entered its site area when the mobile unit is still in its original site area. In addition to transmissions from one site being wrongly processed in another site, similar problems arise when transmissions are incorrectly received on wrong channels in their own site. Such wrong channel reception can arise from interference and intermodulation of channel frequencies being transmitted from a single site.
Briefly, each digitally-trunked RF communications site base station includes a "control" RF channel and multiple "working" RF channels. The working channels are used to carry actual communications traffic, e.g., analog FM, digitized voice, digital data, etc. The RF control channel is used to carry digital control signals between the repeater sites and user RF transceivers (radio units) in the field. When a user's transceiver is not actively engaged in a conversation, it monitors the control channel for "outbound" digital control messages directed to it. User depression of a push-to-talk (PTT) switch results in a digital channel request message requesting a working channel (and specifying one or a group of callees) to be transmitted "inbound" over the RF control channel to the repeater site. The repeater site (and associated trunking system) receives and processes the channel request message. Assuming a working channel is available, the repeater site generates and transmits a responsive "outbound" channel assignment digital message over the RF control channel. This message temporarily assigns the available working channel for use by the requesting transceiver and other callee transceivers specified by the channel request message. The channel assignment message automatically directs the requesting (calling) transceiver and callee transceivers to the available RF working channel for a communications exchange. The control channel is thereafter not used during the communication. Accordingly, the control channel is used to request and assign working channels, and to transmit certain emergency messages.
When the communication terminates, the transceivers "release" the temporarily assigned working channel and return to monitoring the RF control channel. The working channel is available for reassignment to the same or different user transceivers via further messages conveyed over the RF control channel. An exemplary "single site" trunked RF repeater system is disclosed in commonly-assigned U.S. Pat. Nos. 4,905,302 and 4,903,321, which are incorporated by reference.
Single site trunked RF repeater systems may have an effective coverage area of tens of square miles (if the area is free of obstructions) or only a few city blocks (if large buildings obstruct the broadcast coverage of the site and mobile radio units). A single site may include one or more satellite receiving stations, e.g., repeating stations, and a single high power transmitting site if a somewhat larger coverage area is desired. However, the coverage of a single site is often inadequate to reach the broadcast area of some governmental entities and other public service trunking system users that require an RF communications coverage area of hundreds of square miles or of an entire city. To provide large coverage areas, it is necessary to provide multiple RF repeater sites and to automatically coordinate all sites so that a radio transceiver located anywhere in the system coverage area may efficiently communicate in a trunked manner with other radio transceivers located anywhere in the system coverage area.
The control channel of a site may be used when mobile radio units leave one site area and enter an adjacent site area. As a mobile unit roams from one site to another, the mobile radio's reception of the control channel in the first site weakens to such an extent that the radio automatically scans for a second site's control channel. When the control channel for the second site is received, the radio unit sends a log-in message to the second site repeater which logs the new unit into its site and advises the first site, via a multisite network switch, that the unit is to be logged off the first site.
FIG. 1 is a schematic diagram of a simplified exemplary multiple-site trunked radio repeater system, e.g., a multisite system, having three radio repeater (transmitting/receiving) sites S1, S2, and S3 providing RF communications to geographic areas A1, A2, and A3, respectively. Mobile or portable transceivers within area A1 transmit signals to and receive signals from site S1; transceivers within area A2 transmit signals to and receive signals transmitted by site S2; and transceivers within area A3 transmit signals to and receive signals transmitted by site S3. Each repeater site S1, S2, S3 includes a set of repeating transceivers operating on a control channel and plural RF working channels. Each site typically has a site controller (e.g., a digital computer) that acts as a central point for communications in the site, and is capable of functioning relatively autonomously if all participants of a call are located within its associated coverage area.
To enable communications from one area to another a switching network, a "multisite switch" may be provided to establish audio and control signal pathways between repeaters of different sites. These pathways are set up at the beginning of each call and taken down at the end of each call. For example, the site controller (S1) receives a call from a mobile radio in A1 requesting a channel to communicate with a specific callee. A caller requests a channel simply by pressing the push-to-talk (PTT) button on his microphone. This informs the site controller S1 via an "inbound" digital control message transmitted over the RF control channel that a working or audio channel is requested. The site controller assigns a channel to the call and instructs the caller's radio unit to switch from the control channel to the audio channel assigned to the call. This assigned working channel is applicable only within the area covered by that site and is used as a communication channel only for mobile units within the site. Nevertheless, a transmission 110 from a mobile unit in one site may inadvertently be received by a repeater transceiver in a second site, causing the second site to act on the transmission as if it were a transmission from within the site area. The subject of this invention is to avoid having a repeater radio site controller act on a signal from another site unintentionally received by the radio.
In a multisite radio system, when a site controller assigns a working channel for a mobile unit in its site area, the site controller also sends a channel request message to multisite switch (200). The multisite switch also sends the channel request to other site controllers having a designated callee within their site area. Audio signals are routed through the multisite switch such that audio pathways are created to serve one or more callees and one or more dispatcher consoles 202 involved in the communication. Upon receiving a channel request, these "secondary" site controllers (in the sense they did not originate the call) assign an RF working channel to the call for each respective site. Each secondary working channel is operative only in the site area covered by the secondary site controller. The secondary site controller(s) also sends the channel assignment back up to the multisite switch.
A caller can communicate with a radio unit or group of radio units in another area via the multisite switch. The call is initially transmitted to the primary site repeater, routed through an assigned audio slot in the multisite switch to secondary site repeaters, and retransmitted by the secondary sites on various assigned channels in those other areas. When the call ends, the primary site controller deactivates the assigned channel for that site and notifies multisite switch 200 that the call is terminated. The multisite switch propagates an end of call command ("channel drop") to all other site controllers. This releases all working channels assigned to the call and breaks the associated audio routing pathways.
The multisite switch preferably includes a distributed control architecture. The logical functions and computational workload of the multisite switch are shared by various distributed microprocessor "nodes". Each node is connected either to a site controller 102, dispatch console 202, public and/or private landline telephone exchanges and other components of the particular communications system. Most nodes function as switch interfaces and include, for example, Master Interface Modules (MIMs) for nodes coupled to site controllers and Console Interface Modules (CIMs) for nodes coupled to dispatch consoles. Each interface module is supported by a controller card that utilizes several microprocessors. All of the cards have substantially the same hardware and are interchangeable. Each card acts as a gateway interface into the distributed control switch network. Detailed description and operation of such a distributed multi-site switch is set forth in commonly assigned U.S. Pat. No. 5,200,954 to Teel, Jr. et al which is also incorporated herein by reference.
In addition, two or more multisite switches may be linked to form a wide-area multisite system having a relatively-large wide-area covered by a single RF trunked system. As the number of sites in a multisite (or wide area) network increases, the amount of sharing increases of the limited number of channels assigned to that system. While it is preferable that a multisite network not have two sites with overlapping broadcast areas that share the same channel frequencies, such sharing of channel frequencies between adjacent sites may occur. Similarly, a mobile unit assigned to one site may, in certain circumstances (such as when transmitting from a hill on the edge of a site area and in optimum transmission weather conditions) transmit a message that is received by another site that is normally out of range of radio units in that site. Accordingly, there has been a long-felt problem associated with transmissions in one site area being received by a site repeater in another site.
To avoid having transmissions 110 inadvertently received by unintended sites, the site repeating radios were previously geographically isolated to minimize overlapping radio coverage between the sites. Because of graphic isolation, a transmission from one site would not be received on the control channel of another site. Isolation of sites is not always geographically practical as some overlap in radio broadcast coverage almost always occurs in a multisite system. Similarly, another conventional method to avoid having a site repeater radio act on a transmission from another site was to modify the transmission protocols used by the mobile units. Adding site identification information to the transmission protocols usually increases the signalling overhead associated with the transmission (and thereby reduces the available time for transmission of voice or other information) and requires relatively extensive modification of the messaging protocols used by the radios in a multisite network. In addition, most of the site identification protocols increase the difficulty in adapting a message protocol that allows a mobile site to roam from site to site. Moreover, intermodulation of transmissions can generate signals received on another channel in the same site station that can cause problems of transmissions being received and processed on the wrong channel.